Some existing image forming apparatuses may include: a main body having an opening; an image forming unit that is disposed in the main body and configured to form an image on a recording medium; a belt unit detachably attached in an operation position where the belt unit faces the image forming unit in the main body and configured to hold an endless belt rotatably and feed the recording medium; and a pair of lock mechanisms disposed in the main body and configured to fix the belt unit in the operation position by applying an urging force to the belt unit. The belt unit is configured to be inserted from an exterior of the image forming apparatus into the operation position along an attachment direction from the opening of the main body toward the operation position.
In the existing image forming apparatuses, the belt unit includes a pair of pivot shaft portions disposed on a front side relative to the attachment direction. The pivot shaft portions protrude outward in a width direction of an endless belt. The endless belt is wound around a drive roller and a driven roller which are rotatably supported at the front side and a rear side of the belt unit with respect to the attachment direction. The driven roller's shaft protrudes outward in the width direction of the endless belt, and both ends of the shaft function as a pair of engaging protruding portions.
Furthermore, the main body of the existing image forming apparatuses may also include a pair of pivot support portions. When the belt unit is attached to the main body, the pivot support portions are configured to engage the pivot shaft portions of the belt unit so as to support the belt unit such that the belt unit is able to pivot around a horizontal pivot axis that is perpendicular to the attachment direction.
Each lock mechanism of the existing image forming apparatuses is configured to contact a corresponding one of the engaging protruding portions when the belt unit pivots around the pivot axis during attachment, so as to apply tension to the endless belt and fix the belt unit in the operation position.
In the existing image forming apparatuses, to attach the belt unit to the main body, the user holds the rear side (on which a finger engageable portion is disposed near the driven roller) of the belt unit in the attachment direction and inserts the front side of the belt unit through the opening into the main body. After engaging the pivot shaft portions of the belt unit with the pivot support portions of the main body, the user presses the rear side (the finger engageable portion) of the belt unit downward, causing the belt unit to pivot around the pivot axis. At this time, the lock mechanisms contact the respective engaging protruding portions so as to apply an urging force thereto, and the user presses the belt unit downward against the urging force of the lock mechanisms.
In the existing image forming apparatuses, as described above, a distance between a position where each lock mechanism applies the urging force to the corresponding engaging protruding portion (e.g., each end of the driven roller's shaft) and the pivot axis is generally equal to a distance between a pressing position (the finger engageable portion near the driven roller) where the user presses the belt unit to the operation position and the pivot axis. Accordingly, when the user presses the belt unit, the user is more susceptible to the urging force of the lock mechanism, and thus needs to press the belt unit downward with a great force to overcome the urging force. In addition, the tension applied by the lock mechanisms to the endless belt may result in a significantly large force being placed on the belt unit. Similarly, to remove the belt unit, the user may need to raise the belt unit with significant force.